An Orthogonal Frequency Division Multiplexing (OFDM) modulated signal has a high peak-to-average ratio, which requires high linearity in a power amplifier at radio frequency frontend. However, since a terminal has limited power consumption, it is not suitable to obtain high linearity by increasing the static power consumption of the power amplifier. Moreover, as high order Quadrature Amplitude Modulation (QAM) on OFDM sub-carriers is susceptible to non-linearity in both amplitude and phase of the amplifier, a pre-correction method is needed such that the terminal power amplifier can output linear amplification signals ultimately.
In a Multiple Input Multiple Output (MIMO) terminal, multiple paths of radio frequency signals are coupled with each other, which results in advance of power compression, multi-tone inter-modulation and increase in adjacent channel leakage ratio of the power amplifier. Particularly, adjacent channel rejection ratio and linearity are also deceased and even it cannot satisfy requirements on bandwidth (100 MHz) and rate (×100 Mbps) by International Mobile Telecommunication (IMT)-Advanced. Thus, it is necessary to increase linearity by using a pre-corrector. In addition, each pre-corrector in a MIMO system requires a down-mixer. These down-mixers should be multiplexed on a terminal in order to save space and to reduce system complexity. Therefore, the design of the MIMO radio frequency frontend to achieve high linearity and low crosstalk in a terminal becomes a problem to be solved.
There is an existing solution where each of radio frequency links at radio frequency frontend is subjected to feedforward on an individual path, or further includes pre-distortion correction for feedback detection.
However, this solution has the following drawbacks:
1) Repetitive updates for pre-correction vector are required for a number of times. This approach is resource consuming, which leads to insufficient operation bandwidth of the corrector and failure to satisfy the requirement of 100 MHz by IMT-Advanced. The corrector needs to store input and output error vectors in a lookup table and to address, in accordance with the power of an input signal, the corresponding correction vector, so as to modify the input signal. Thus, the operation bandwidth of the corrector depends on a speed of updating and addressing of the lookup table. Due to a high code rate of broadband high speed communication, sequential detection of feedback signals for each time according to this method may cause many times of repetitive detection, occupation of resource consumption and thus decrease in addressing speed, such that the bandwidth of the high speed corrector for generating the correction vector is only about 30 MHz.
2) The linearity parameter cannot satisfy the requirement by IMT-Advanced. High speed broadband communication requires very high adjacent channel rejection ratio (−45 dBc) which has the following relationship with third-order inter-modulation:ACLRn=(2×[(Ptot−3)−OIP3])+Cn   (1)where
Ptot=a total output power of all carriers, in dBm,
OIP3=third-order inter-modulation of the device, in dBm,
ACLRn=ACLR for the n-th carrier, in dBc,
Cn=deduction amount for performance degradation when n signals are input simultaneously, in dBc, and
C1-3, C2-9, C3-11, C4-12, C9=13.
When the input power reaches a typical value of 30 dBm, the third-order inter-modulation is required to be −51 dB, which is not achievable by existing pre-correctors.
3) For a MIMO terminal equipped with multiple-path radio frequency frontend, the input coupling is equivalent to input of a number of signal, which results in advance of output power compression and increase of inter-modulation component.
There is another existing solution where two matrix filters for dividing signal power uniformly are mounted respectively in front of and behind a multi-path amplifier, such that the peak power and the average power of the signal are close to each other. In this way, the requirement on linearity of the power amplifier at radio frequency frontend can be reduced.
However, this solution has the following drawbacks:
1) The peak power can only be reduced to 1/N of its own, where N is the number of radio frequency links and is generally less than 6. Thus, the resultant increase in linearity is less than 21 dBc. For a terminal with less than 4 number of antennas, the high linearity requirement by IMT-Advanced system cannot be satisfied.
2) The matrix filter is too large in size to be employed in a terminal.
3) The imperfect characteristics of the matrix filter may damage the premise based on which MIMO communication can establish, that is, low correlation among space division sub-channels.